Low carbon, high speed metal core wire

ABSTRACT

The present invention is a weld wire comprising a sheath encapsulating a metal core made of powdered metal, wherein a fill percentage of the metal core is no less than approximately 12%. The metal core comprises a core composition alloyed with an alloying element or an combination of elements comprising Cr, Mo, V, W, Hf and Nb or combinations thereof, wherein a total weight percentage of the alloying element or the combination of elements in the core composition does not exceed approximately 1%. In a particular embodiment, the alloying element is Mo in the amounts selected from the range of about 0 to about 0.5 percent by weight and the fill percentage of the metal core is selected from the range of about 12% to about 30%. In a particular embodiment of the invention, the total percentage of the combination of elements is selected from the range of about 0.4% to about 0.8%.

RELATED APPLICATIONS

This application is a Continuation of Ser. No. 09/683,523 filed on Jan.13, 2002 and issued as U.S. Pat. No. 6,787,736 on Sep. 7, 2004 which isincorporated by reference herein in its entirety.

BACKGROUND OF INVENTION

The present invention relates generally to gas metal arc welding. Moreparticularly, the present invention relates to a composition andproperties of a family of consumable electrodes for gas metal arcwelding of carbon steels.

The growing demand for increased electric arc welding productivity callsfor continuing efforts to reduce welding time while improvingproductivity of structural fabrication, especially for the roboticapplications. In order to operate a robotic welder at its maximumcapacity, welding consumables should be able to provide good weldingbeads at the maximum travel speed without increasing the number ofwelding defects. One of the approaches to improve the productivity andreduce the welding time would be to increase the deposition rate andtravel speed for a given weld size. Unfortunately, it often happens thatan increase in a travel speed of a welding process is accompanied by anincrease in the number of welding defects.

Another approach to solving the problem is to try to manipulate thecomposition and structure of a welding electrode to change itsproperties in such a way that the deposition rate and travel speed of awelding process will increase advantageously. The electric arc weldingprocess often uses metal consumable electrodes in the form of tubularwelding wires. These metal core welding wires are usually made ofgenerally tubular composites having a metal sheath and a core made ofvarious powdered materials. The known tubular types of wires can beclassified as metal core wires or flux-core wires. Metal core wireelectrodes, with relatively simple chemical composition and knownmetallurgy, have a high deposition rate and high deposition efficiencywhile producing less slag, and, therefore, are increasingly used as analternative to solid or conventional flux core wires for improvedproductivity in structural fabrication. If higher deposition ratesexhibited by metal core wires could be combined with a high travel speedand a good quality welding bead, a welding electrode having such a metalcore wire would greatly increase the productivity of arc welding used inmany applications in the automotive, shipbuilding and generalfabrication industry.

Conventional metal core wires of the type described in a number of U.S.Patents relate to different types, structures or combinations ofelemental composition of welding electrodes. For example, U.S. Pat. No.3,656,918, relates to an alloy suitable for use as a weld-fillermaterial with about 2% of Mo as one the alloying elements in combinationwith Cr and Ni. U.S. Pat. No. 3,635,698 relates to a weld-filler metalmade of a low alloy steel alloyed by a combination of Ni, Cr and Mo.U.S. Pat. No. 4,782,211 relates to a cluster welding electrode assemblyhaving a rod covered with a flux alloyed by defined amounts of Mo and W.That Patent mentions that controlled amounts of tungsten, preferably inthe form of a ferrotungsten alloy, appear to modify the workingcharacteristics of the weld metal. U.S. Pat. No. 5,523,540 relates to awelding electrode of a composition within certain elemental compositionranges. The welding electrodes of that Patent form weld deposits with alow carbon bainitic ferrite microstructure of sufficient strength forwelding high-strength steels. U.S. Pat. No. 5,824,992 relates to ametal-core wire with a core composition between approximately 2.5-6.0%or 2.5-12% of the total weight of the metal-core wire.

Manufacturing of a metal core wire normally involves forming, fillingand then drawing or rolling the wire. First, a steel sheath is formedand bent into a U-shape tube, then an amount of metal powder, such asiron powder, is fed into the U-shaped tube. The subsequent forming anddrawing process encloses and compacts the powder to form a wire andreduces that wire to its final shape. If an improved metal-core wirewith a higher deposition rate and travel speed manufactured as describedabove could be provided, the productivity of the robotic arc welderscould be significantly increased.

It is therefore an object of the present invention to provide a metalcore wire and a method of manufacturing such a wire allowing an 15-20%increase in the deposition rates.

It is also an object of the present invention to provide a compositionof a metal core wire electrode leading to a 45-50% increase in thewelding electrode travel speed.

It is also an object of the present invention to provide carbon steelcomposite core wires exhibiting a combination of high melting points andappropriate surface tension of the molten wire, which combination leadsto a high deposition rate and travel speed in the welding process.

SUMMARY OF INVENTION

The present invention is a weld wire comprising a sheath encapsulating ametal core made of powdered metal, wherein a fill percentage of themetal core is no less than approximately 12%. The metal core comprises acore composition alloyed with an alloying element or an combination ofelements comprising Cr, Mo, V, W, Hf and Nb or combinations thereof,wherein a total weight percentage of the alloying element or thecombination of elements in the core composition does not exceedapproximately 1%. In a particular embodiment, the alloying element is Moin the amounts selected from the range of about 0 to about 0.5 percentby weight and the fill percentage of the metal core is selected from therange of about 12% to about 30%. In a particular embodiment of theinvention, the total percentage of the combination of elements isselected from the range of about 0.4% to about 0.8%.

To achieve a 15-20% increase in the deposition rates of the wires, thepresent invention provides a sheath encapsulating a steel core having acore fill percent of more than 12% and the steel core having acomposition comprising an alloying element selected from the groupconsisting of Cr, Mo, V, W, Hf and Nb and combinations thereof. Thedeposition rates of such wires increases with the increase of the corefill percent, when the wire is used in the welding process. Inparticular, the deposition rate increases from about 15 lb/hr for thecore fill percent of about 12% to the deposition rate of about 20 lb/hfor the core fill percent of about 30%. In particular embodiments, atotal weight percentage of Mo varies from about 0% to about 0.4%, and atotal weight percentage of an alloying element does not exceedapproximately 1%.

To achieve a 40-50% increase in the travel speed, the wire of thepresent invention comprises a sheath encapsulating a metal core, whereina core fill percent of the metal core is higher than 12%, the metal corehaving a composition alloyed with an alloying element or an combinationof elements comprising Cr, Mo, V, W, Hf and Nb or combinations thereof,wherein a total weight percentage of the alloying element or thecombination of elements in the core composition does not exceedapproximately 1%, and wherein a travel speed of the weld wire when usedin welding ranges from about 65 in/min to about 145 in/min. The travelspeed of the wire when used in welding can be characterized via amaximum travel speed ranging from about 80 in/min to about 145 in/minfor the core fill percent ranging from about 12% to about 30%. Themaximum travel speed of the wire measured during the welding experimentsof the present invention corresponded to the composition comprising apercentage of Mo ranging from about 0% to about 0.4%.

A method of manufacturing a weld wire comprises forming a sheath into ashape which can be filled with a metal powder; filling the sheath withthe metal powder, the metal powder having a composition alloyed with analloying element or an combination of elements comprising Cr, Mo, V, W,Hf and Nb or combinations thereof, wherein a total weight percentage ofthe alloying element or the combination of elements in the corecomposition does not exceed approximately 1%. Further in themanufacturing process the metal powder is compacted to form a metalcore; and the wire is drawn to achieve a core fill percentage of themetal core no less than 12%. According to the invention, the core fillpercentage ranges from about 12% to about 30%, the alloying element isMo ranging from about 0% to about 0.4%. The total weight percentage ofthe combination ranges from about 0.4% to about 0.8%.

These and other objects, features and advantages of the presentinvention will become more fully apparent upon consideration of thefollowing detailed description of the invention with the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of a metal core wire.

FIG. 2 is a graph illustrating the voltage-wire feed speed relationship.

FIG. 3 is a graph illustrating the relationship between deposition ratesand core fill percentages.

FIG. 4 is a graph illustrating the relationship between transferfrequencies and core fill percentages.

FIGS. 5(a)-(h) are the photos of the welding specimens provided in Table5.

DETAILED DESCRIPTION

The present invention comprises is based on a metal core wire of a mildsteel metal core of Fe, Mn and Si, wherein Mn and Si are the mainalloying elements. A more detailed composition of the wire is providedin Table 1, which composition corresponds to the E70C-6C type electrodeof the American Welding Society (AWS) specification A5.18.

Table 1. Conventional low carbon steel metal core electrode weld metalcomposition (AWSA.5.18).

[t1] TABLE 1 Elemental Composition AWS A5.18 Mn Up to 1.7 Si Up to 0.9Cu Up to 0.5 S Up to 0.03 P Up to 0.03 Fe Balanced

The main function of Mn and Si in such wires is to deoxidize the metaldeposited during the welding process and to modify the microstructure ofthe deposited metal to obtain the desired mechanical properties.Additionally, Si increases bead wetting, therefore, improving theweldability of the deposited wire. As provided in Table 1, small amountsof metallic and non-metallic additions (Cu, S, P) to the metal powderstabilize the arc and reduce the amount of diffusible hydrogen duringthe welding process. The metallic fraction in a wire electrode isnormally no less than about 95 Wt % of the total weight of theelectrode, while the weight of the core composition falls within to therange of about 10 Wt % to about 20 Wt % of the total wire weight.

The metal core wire of the present invention is characterized by aspecific elemental composition which reduces the surface tension of themolten weld metal of the composite wire described in the previousparagraph. The reduction in the surface tension occurs due to thealloying of the core composition of the wire with a single addition orcombination of Cr, Mo, V, W, Hf and Nb. The reduced surface tensionhelps spread the molten weld metal in the arc gouge area—the portion ofa base metal that has been melted by the arc, but which has not beenfilled with the molted weld or a filler metal yet. A composition for themetal core electrode wire of the present invention is provided in Table3.

Table 3. (The total percentage of Cr, Mo, W, V, Hf, and Nb not to exceed1%).

Elemental composition for the metal core electrode weld metalcomposition of the present invention.

[t2] TABLE 3 Mn Up to 1.7 Si Up to 0.9 Hf Up to 0.5 Cr Up to 0.5 Mo Upto 0.5 W Up to 0.5 V Up to 0.5 Nb Up to 0.5 Cu Up to 0.5 S Up to 0.03 PUp to 0.03 Fe Balanced

Another reason to alloy the metal core wire electrode with a singleaddition or a combination of Cr, Mo, V, W, Hf and Nb is the increase inthe welding speed occurring due to the increase in the freezing range ofthe molten weld (and, therefore, the decrease in the solidificationrates). Since high travel speeds lead to high solidification rates, themolten weld is usually can not completely fill the gouge due to aninsufficient time available for such filling, resulting in the creationof undercuts, humps and other defects that can be formed in the arcgouge area. Increasing the freezing ranges of the wires, as achieved inthe present invention, improves the wetting of the base metal and slowsdown the solidification process, allowing enough time for the moltenweld to better fill the gouge. Examples of the experiment wires andtheir compositions are provided in Table 2. In particular, theexperimental wires are alloyed with various percentages of Mo (about 0%,about 0.2% and about 0.4%) while having fill percentages varying from12% to 30%.

Table 2: Experimental wires (Dia-1.4 mm) and their compositions:

[t3] TABLE 2 Fill % C Mn Si Ni Mo Test 067 12 0.041 1.437 0.549 0.0180.026 Test 068 18 0.028 1.394 0.540 0.028 0.027 Test 069 24 0.025 1.6930.662 0.032 0.032 Test 070 30 0.024 1.018 0.325 0.033 0.030 Test 066 120.043 1.314 0.497 0.016 0.219 Test 056 18 0.033 1.256 0.456 0.022 0.196Test 058 24 0.027 1.183 0.428 0.023 0.206 Test 059 30 0.031 1.320 0.4560.021 0.192 Test 071 12 0.021 1.278 0.419 0.016 0.403 Test 072 18 0.0391.246 0.456 0.019 0.421 Test 073 24 0.036 1.275 0.489 0.016 0.418 Test074 30 0.035 1.173 0.414 0.020 0.415

It should be understood that while the presented experimental resultspertain to alloying the test wires with Mo, it has been established thatalloying the wires with single additions or combinations of Cr, Mo, V,W, Hf and Nb produces experimental results comparable to the onesprovided for Mo. Usually the weight percent of the alloying combinationdid not exceed 1%, and in most cases the weight percent of the alloyingcombination was within the range from about 0.4 to about 0.8%.

The high fill percentages of the alloying elements in the experimentalwires of Table 1 are significant for achieving improvement of both thedeposition rate and the wetting action of the molten metal. Thedeposition rates of 12 different test wires according to thecompositions listed in Table 2 were measured for the 1.4 mm diameterexperimental wires at 350 A, 34V, DCEP, depending on the fill percentageof the wires. The fill percentages of the wires in Table 2 varied fromabout 12% to about 30% for the series of wires alloyed with about 0%,0.2% and 0.4% of Mo respectively.

FIG. 3 illustrates the results of these measurements of the depositionrates for experimental wires 067-070, corresponding to the compositionswith almost 0% of Mo. As follows from FIG. 3, the deposition rate ofwire 070 with about 30% core fill is about 20 lb/h, which is much higherthan the deposition rate of about 15.6 lb/h of wire 060 with only about12% core fill. The observed increase in the deposition rate happens dueto the increase in electrical resistance of the wire with a higher corefill percent. More specifically, the higher core fill provides a higherproportion of the metal powder material in the cross-section of the wirerelative to the total cross-section of the wire. Since the powder metalcore wire exhibits higher electrical resistance than the wire of thesame diameter with a solid metal core, the higher fill metal powder corewire generates more heat during the welding process and, therefore,leads to the higher deposition rates. It is noted that in the context ofthe present invention the term “solid metal core” means a metal coremade of bulk metal and not of a compacted metal powder.

In order to provide consistent results in welding processesdemonstrating high deposition rates and high travel speed, it isimportant to have a way to determine and select the optimized weldingparameters for such processes. The parameters of interest in theelectrical arc welding with consumable electrodes are wire feed speed,voltage and travel speed. If non-optimized welding parameters areselected, then a high defect rate and low quality of welds are likely tooccur. For a fixed size fillet welding process, which uses constantvoltage, the wire feed speed is usually proportional to the wire travelspeed, assuming that the deposition efficiency is constant. Depositionefficiency refers to a percentage of weight of the wire that is actuallydeposited on the base metal and not lost in slag, sputter or flume. Formost metal core consumable welding wires the deposition efficiency isusually in the range of 90-98%, which may vary slightly during thewelding process. Considering an a illustrative example, for a {fraction(1/8)} inch joint fillet weld, the travel speed equals about 20-25% ofthe wire speed and an approximately constant voltage. FIG. 2 providesthe data on the wire feed speed and voltage for a number of listedspecimen. FIG. 2 illustrates the relationship between the optimizedvoltage and a wire feed speed used in the experiments of test wires for1.4 mm wires at a fixed arc length of approximately {fraction (1/8)}″.Other welding parameters used to obtain the results of FIG. 2 are ⅛″ lapjoint, DCEP, 100% CO.sub.2 shielding gas, 1 F electrode position at 15degrees downhill orientation. The length of the arc was calibrated usinga high speed CCD camera. The quality of the welds obtained after thewelding process was performed on the specimens in Table 5 is illustratedin FIGS. 5(a)-(h).

An additional way of illustrating the advantage of high core fill metalwires is to analyze their droplet transfer frequency during the weldingprocess. The droplet transfer frequency is a major index used inevaluating the stability of the arc. A higher droplet transfer frequencyresults in a better stability of the arc, as well as in a betterconvection inside the weld pool, enhanced wetting action of the liquidmetal and a reduced spatter level of the wire. For the test wires067-070 of Table 2, the droplet transfer frequency has significantlyincreased with the increase of the core fill percentage of the wires, asillustrated in FIG. 4. The droplet frequency data of FIG. 4 wereobtained with a high speed CCD camera, the measurements were taken at350A, 43V, DCEP).

As follows from FIG. 4, the overall weldability of the wires is improvedby increasing their core fill percentage and using the wires of thechemical composition described in the present invention. To furtherillustrate this finding, Table 4. lists the data showing the increaseddeposition rates and welding speeds of the welding processes using theexperimental wires. The experiments were run for different fillpercentages of the series of wires having about 0%, 0.2% and 0.4% of Mo.

Table 4. Deposition rate and maximum travel speeds of the experimentalwires.

[t4] TABLE 4 Depositition Rate Maximum travel speed % Mo Fill % (lb/hr)(in/min) Test 0.026 12 15.6 80 067 Test 0.027 18 16.7 80 068 Test 0.03224 18.7 80 069 Test 0.030 30 20.0 100 070 Test 0.219 12 15.4 95 066 Test0.196 18 15.8 100 056 Test 0.206 24 18.2 140 058 Test 0.192 30 19.5 145059 Test 0.403 12 16.0 95 071 Test 0.421 18 17.1 110 072 Test 0.418 2419.0 140 073 Test 0.415 30 20.8 140 074

Travel speed measurements in Table 4 were performed according toAWS/ANSI D8.8-89 (SAE HS J1196): Specification for Automotive and LightTruck Components Weld Quality—Arc Welding, which specification isincorporated herein by reference. Test wire 067 in Table 4 is an exampleof a conventional metal core wires with 10-20% core fill percent and noalloying element of more than 0.1% other than Mn and Si. Its depositionrate is about 15.6 lb/h with a maximum travel speed of 80 in/min. As itcan be clearly seen in Table 4, test wire 058, which is an example ofthe present invention, exhibits a higher deposition rate of 18.2 lb/hwith a much higher deposition speed of 140 in/min (maximum travelspeed). Noticeably, the best deposition rate and travel speed wereexhibited by the test wires combining a high core fill percent with anaddition of a controlled amount of the alloying element, Mo.

Manufacturing of a metal core wire of the composition and high fillpercentage in accordance with the present invention involves providing asheath (such as a steel sheath) and forming (often bending) the sheathinto a shape that can be filled with a metal powder, such as, forexample; iron powder. Often such a shape would be a U-shape. The metalpowder has a composition alloyed with a single addition or a combinationof Cr, Mo, V, W, Hf and Nb, which is fed into the sheath. The percentageof the combination of the alloying elements usually does not exceed 1%,while the preferred percentage falls within the range between about 0.4%and about 0.8%. When Mo is used as a single alloying element, itspercentage varies from about 0% to about 0.4%. The subsequent formingand drawing process encloses and compacts the powder to form a wire andreduce that wire to its final shape with the core filled percentageshigher than 12%, as described earlier.

It has been, therefore, demonstrated that an addition of up to 0.4% ofMo to a metal core electrode wire having a core fill percent of morethan 12% significantly increases its deposition rate and travel speedwithout sacrificing the quality of the resulting weld, resulting in asignificant increase of productivity of the welding processes. Thewritten description of the invention enables one skilled in the art tomake and use what is at present considered to be the best mode of theinvention, and it should be appreciated and understood by those skilledin the art that the existence of variations, combinations, modificationsand equivalents falls within the spirit and scope of the specificexemplary embodiments disclosed herein. It is also to be understood thatthe illustrative examples described herein are not to be construed aslimiting the present invention in any way. The objects, features andadvantages of the present invention as claimed in the appended claimsare applicable to all types of metal core wires, such as low carbonmetal core, stainless steel metal core and low alloy metal core wires.

1. A weld wire comprising: a sheath encapsulating a metal core made ofpowdered metal, wherein a fill percentage of the metal core is higherthan approximately 12%; and the metal core having a core compositionalloyed with an alloying element selected from the group consisting Cr,Mo, V, W, Hf. Nb and combinations thereof, wherein a total weightpercentage of the alloying element in the core composition does notexceed approximately 1% wt.
 2. The weld wire of claim 1, wherein thealloying element is Mo in an amount selected from the range of about 0to about 0.5 percent by weight.
 3. The weld wire of claim 1, wherein thefill percentage of the metal core is selected from the range of about12% wt to about 30% wt.
 4. The weld wire of claim 1, wherein the totalpercentage of the alloying element is selected from the range of about0.4% wt to about 0.8% wt.
 5. The weld wire of claim 1, wherein the corecomposition mainly comprises, approximately, by weight: C 0.021-0.043%,Mn 1.0-1.69.0%, Si 0.33-0.66%, and Ni 0.016-0.033% and the fillpercentage of the metal core is higher than approximately 12%.
 6. Theweld wire of claim 1, wherein the alloying combination comprises,approximately, Cr Up to 0.5% wt Mo Up to 0.5% wt W Up to 0.5% wt V Up to0.5% wt Hf Up to 0.5% wt Nb Up to 0.5% wt.
 7. An improved productivityweld wire comprising: a sheath encapsulating a steel core having a corefill percent of more than 12%; and the steel core having a compositioncomprising an alloying element selected from the group consisting of Cr,Mo, V, W, Hf. Nb and combinations thereof, wherein a deposition rate ofthe weld wire when used during welding increases with the increase ofthe core fill percent.
 8. The improved productivity weld wire of claim7, wherein the deposition rate increases from about 15 lb/hr for thecore fill percent of about 122% wt to the deposition rate of about 20lb/h for the core fill percent of about 30%.
 9. The improvedproductivity weld wire of claim 7, wherein a total weight percentage ofan alloying element does not exceed approximately 1%.
 10. The improvedproductivity weld wire of claim 8, wherein a total weight percentage ofMo varies from about 0% to about 0.4%.
 11. The improved productivityweld wire of claim 7, wherein the steel core is made of a compactedmetal powder.
 12. A method of manufacturing a weld wire comprising:forming a sheath into a shape which can be filled with a metal powder;filling the sheath with the metal powder, the metal powder alloyed withan alloying element selected from the group consisting of Cr, Mo, V, W,Hf, Nb and combinations thereof, wherein a total weight percentage ofthe alloying element in the core composition does not exceedapproximately 1% wt; compacting the metal powder to form a metal core;and drawing the wire to achieve a core fill percentage of the metal coreno less than 12%.
 13. The method of claim 12, wherein the core fillpercentage ranges from about 12% wt to about 300% wt.
 14. The method ofclaims 13, wherein the alloying element is Mo and wherein the totalweight percentage of Mo ranges from about 0% to about 0.4%.
 15. Themethod of claim 12, wherein the total weight percentage of thecombination ranges from about 0.4% to about 0.8%.
 16. The method ofclaim 12, wherein the alloying combination comprises, approximately,Cr—Up to 0.5% wt Mo—Up to 0.5% wt W—Up to 0.5% wt V—Up to 0.5% wt Hf—Upto 0.5% wt Nb—Up to 0.5% wt.